I. Field of the Invention
This invention relates to alpha-olefin polymers used as lubricant viscosity index improvers. The invention is also directed to lubricant compositions containing at least one alpha-olefin polymer of the present invention.
II. Description of the Related Art
It is well known in the art that the viscosity of lubricating oils decreases with increasing temperature. A commonly-used measure of the change of the viscosity of the oil with the temperature is an empirical quantity called viscosity index. The viscosity index (VI) for any given oil is derived by measuring the viscosity of the oil at 40.degree. C. and 100.degree. C. and then calculating the viscosity index from detailed tables published by the ASTM (ASTM Standard D 2270). Generally, oils having a viscosity index of about 80 or 90 are desirable for practical applications. Such oils are composed usually of saturated paraffinic and alicyclic hydrocarbons which provide long life, freedom from sludge and varnish and perform satisfactorily when combined with specific additives for a given practical application (e.g., see Kirk-Othmer "Encyclopedia of Chemical Technology", Third Edition, John Wiley and Sons, 1981, Volume 14, pages 488-491).
Additives used to improve the viscosity index of a given oil are also well known in the art. Such additives are commonly produced from polyisobutylenes, polymethacrylates and polyalkylstyrenes, and they are used in the molecular weight range of about 45,000 to about 1,700,000. Viscosity index improvers appear to function primarily by increasing the viscosity of a given lubricating oil without changing the original viscosity-temperature coefficient or relationship. The use of such VI improvers is of particular importance in petroleum oils whose lower viscosity fractions from a petroleum crude have by far the lowest viscosity-temperature coefficients. The thickening effect of the additive can produce an oil having a viscosity index of 50 units or more above the value obtained with a higher molecular-weight fraction from the same crude. In addition to polyisobutylenes, polymethacrylates and polyalkylstyrenes, polymerized alpha-olefin polymers have also been used in the past for improving the viscosity properties of lubricating oils. For example, Heilman et al, U.S. Pat. No. 3,795,616, teach the preparation of isotactic alpha-olefin polymers prepared from straight or branched chain alpha-olefin monomers having from 5 to 12 carbon atoms. The polymers are prepared by polymerizing the monomer in the presence of a Ziegler-Natta type catalyst which is useful for the polymerization of propylene. Particularly useful are catalysts containing titanium and vanadium salts in conjunction with aluminum alkyls and alkyl chlorides.
Heilman et al, U.S. Pat. No. 3,965,018, disclose a process for preparing a concentrate of a poly-alpha-olefin in a lubricating oil base, comprising conditioning a previously-hydrotreated lube oil base stock in the presence of hydrogen and a Ziegler-Natta catalyst. Subsequently, a higher alpha-olefin, such as alpha-olefin of 6 to 20 carbon atoms or a mixture of alpha-olefins of 6 to 12 carbon atoms and 16 to 20 carbon atoms, is polymerized in the conditioned, hydrotreated oil.
Heilman et al, U.S. Pat. No. 4,132,663, disclose the preparation of pour point depressants for a lubricating oil, comprising copolymerizing 1-hexene and 1-octadecene in the presence of a Ziegler-Natta catalyst.
Giannetti et al, U.S. Pat. No. 3,637,503, disclose lubricating oil compositions having good shear stability obtained by blending a mineral lubricating oil and a viscosity-index improving amount of a polymer of a normal alpha-olefin having 4 to 16 carbon atoms per molecule. The polymers are prepared in the presence of aluminum chloride catalyst at a temperature of about -40.degree. to about +70.degree. F.
Culter et al, U.S. Pat. No. 3,692,676, and Mack et al, U.S. Pat. No. 4,358,572, disclose methods of reducing friction loss of non-aqueous petroliferous liquids by adding to the liquids alpha-olefin polymers prepared by polymerizing alpha-olefin monomers of 6 to 20 carbon atoms in the presence of Ziegler-Natta catalysts.
Rossi et al, U.S. Pat. No. 3,790,358, Rossi et al, U.S. Pat. No. 3,926,579, Morduchowitz et al, U.S. Pat. No. 4,022,590 and Rossi et al, U.S. Pat. No. 4,088,589, disclose the use of higher alpha-olefin copolymers in lubricating compositions for a variety of purposes.
However, a need still exists in the art for an effective viscosity-index improving composition which can be used in very small amounts in a lubricating oil composition to improve the viscosity characteristics of the lubricating oil composition.